Analysis systems, which operate using photometric signals and in which test carriers tailored to the analysis devices are used, are employed to determine an analyte in an analysis liquid, in particular in bodily fluids such as blood for the purpose of medical testing. The test carriers (which are also referred to as analysis elements) are usually implemented in form of test strips. They typically have one or more test panels, which comprise at least one test layer. The test layer contains one or more reagents. When an analysis liquid is applied, a chemical reaction of the constituents of the analysis liquid occurs, which results in a detectable change of the test panel, in particular in a color change. This change is analyzed using suitable methods. For example, in a reflection photometry measurement, the concentration of a constituent of the analysis liquid to be determined may be concluded from the diffuse reflectance of the test panel after completion of the chemical reaction. In other cases, the desired result may be derived from the chronological change of the reflectivity.
Other analysis systems operating using optical analysis are also known, in particular systems with test carriers which are implemented as capillary test strips. In this case, the reagents are located in a capillary channel which also has an optical measuring zone. Moreover, systems having optical reference channels are also known, as well as systems which operate on the base of fluorescence measurements.
The system has a test carrier, which has an optical evaluation zone. The system comprises a light source for illuminating the optical evaluation zone on the test carrier, a signal generator for generating a first control signal the first control signal having a frequency F1 and an intensity I1, and a light sensor for receiving light remitted from the optical valuation zone, and for converting it into a measuring signal. The measuring signal and the first control signal of the signal generator are fed to a first frequency-selective amplifier. A first output signal is outputted at the output of the amplifier.
Furthermore, the present invention is directed to a method for acquiring and evaluating optical signals. Light received by a light sensor is converted into a measuring signal, which is fed to a frequency-selective amplifier.
The present invention is directed to optical analysis systems of arbitrary construction, insofar as they use test carriers having an optical evaluation zone, in which an optically measurable change is detected using light-optical measuring methods as a measure of the desired analytical result. These are primarily photometric methods, in which light irradiated onto the optical evaluation zone is diffusely reflected. However, the present invention is also additionally suitable for fluorescence measurements. The optical evaluation zone is also referred to in many cases as a “test panel”. This term is also used in the following to identify the optical evaluation zone without restricting the generality.
The measurement and evaluation of the optical signals demands especially high requirements on the precision, to determine the very small measuring currents or voltages with sufficient resolution and allow a quantitative analysis. The measurements are influenced by multiple interference sources, which include the typical problems when measuring small signals. These are, for example, amplifier drift or superimposed DC or AC voltages, in particular low-frequency interfering voltages, as well as interfering currents of various types.
In addition to these interferences, optical interferences due to external light exist. These include constant external light components and modulated interferences, in particular from utility-operated external light sources, which act at the typical utility frequencies and their harmonic.
Analysis devices having test carriers, which reduce the entry of external light into the measuring area of the device, in particular onto the optical evaluation zone, by constructive measures, are known. These devices have a narrow channel, in which the test carrier is inserted into the housing interior. Another embodiment has a cover which is closed after introduction of the test strip and before beginning the measurement.
In addition to the constructive mechanical suppression of the interfering light, systems are known in which an electronic suppression of the interference by external light is performed. For example, WO 01/22871 A1 discloses an optical glucose measuring system, in which a frequency-selective amplifier in the form of a lock-in amplifier is used. On one hand, the lock-in amplifier is fed by the measuring signal of a magneto-optical sensor. On the other hand, it receives a signal of an external signal generator having a predefined frequency as a reference signal. The signal component of the measuring signal whose frequency corresponds to the reference signal of the signal generator is thus selectively amplified. Filtering for a specific frequency is thus performed by using the lock-in amplifier. Interfering light having frequencies, which differ from the frequency of the reference signal, is not amplified or is only amplified strongly damped and is thus not taken into consideration or is only taken into consideration correspondingly reduced in the measurement. To further improve this system, it is suggested that two lock-in amplifiers be connected in series. The output signal of the first lock-in amplifier is thus fed to the second lock-in amplifier. The signal to noise ratio of the sensor is thereby further improved. However, if an interference has the frequency of the reference signal, the interfering signal is also amplified and an incorrect measurement result is output. The occurrence of interference and the corruption of the measurement results can not be recognized by the system.
The use of lock-in amplifiers and the combination of multiple amplifiers, as well as a cascaded serial connection of two amplifiers is known, for example, from “Application Note AN1003, Low Level Optical Detection using Lock-In Amplifier Techniques” from “AMETEK Signal Recovery”.
Another pathway is followed by the methods disclosed in U.S. Pat. No. 5,463,467 (DE 43 21 548 A1) and U.S. Pat. No. 4,553,848 (DE 3138879 A1) for acquiring and evaluating analog photometric signals, in which the test panel of a test carrier is irradiated by a light source cycled in light-dark phases. The remitted light is recorded and integrated by a measuring receiver during a measuring period comprising multiple light-dark phases. Effective interference and external light suppression, which allows measurement even without the shielding against ambient light typical up to this point, is achieved by an irregular sequence of the light and dark phases integrated during the measuring period. The sequence is irregular in such a manner that the Fourier-transformed frequency spectrum contains multiple different frequencies, so that each individual frequency enters into the measuring result as a small fraction. Corresponding interfering frequencies can thus act on the corruption of the measuring result with a fractional error contribution.
The present invention provides suppression of interfering signals in the measurement and evaluation of optical signals to determine an analyte in an analysis liquid and to increase the reliability of the determination.